1. Field of the Invention
This invention relates to a catalyst for removing nitrogen oxides and a process for producing the same. More particularly it relates to a catalyst for removing nitrogen oxides having a high strength and suitable for a molded member of a large-sized catalyst structure.
2. Description of the Related Art
As a catalyst for removing nitrogen oxides (hereinafter abbreviated to denitration catalyst), that obtained by molding a catalyst composition comprising titanium oxide (TiO.sub.2) and at least one of oxide of molybdenum (Mo), tungsten (W), vanadium (V), etc. into granular form, plate form, honeycomb form or the like have generally been used. In the case of treating exhaust gas from a boiler furnace in which a fuel such as heavy oil, coal, etc. is burnt, since the gas contains a large quantity of soot and ashes, the denitration catalyst to be contacted with the exhaust gas is desired to have a low pressure loss and hardly deposit such soot and ashes. From this point of view, a catalyst assembly of plate-type catalyst, a honeycombform catalyst having a high percentage opening, etc. have been used. These catalysts have flow paths of the gas formed in parallel to the flow direction of the gass.
As such catalysts, a number of catalysts such as a catalyst obtained by coating a metal plate substrate with a catalyst component (Japanese patent publication No. Sho 61-28377/1986), a catalyst obtained by extrusion-molding a catalyst component into a honeycomb form (Japanese patent publication No. Sho 60-3856/1985),
a catalyst obtained by molding a ceramic fiber mat or paper into a honeycomb form, and coating the resulting material with a catalyst precursor (Japanese patent publication No. Sho 58-11253/1983), etc. have been known and practically used.
Among the above prior art, the catalyst obtained by coating a metal plate with a catalyst component is superior in that the flat portion of the catalyst is so large that the pressure loss becomes small; hence ashes are hardly deposited, but there have been raised drawbacks that the resulting catalyst is heavy and the metal plate is liable to be oxidized.
Further, in the case of a catalyst obtained by molding a catalyst component into a honeycomb form according to extrusion-molding process, the resulting product is restricted to a dimension of about 150mm square or less due to a limit of the molding technique. Hence, in order to pack such a catalyst product in a denitration apparatus having a large capacity such as several hundreds m.sup.3, it has been necessary to use a large number of such catalysts. Further, the molded product has a low impact strength. Moreover, the catalyst obtained by coating the surface of the ceramic fiber mat or sheet with a catalyst component has a drawback that its mechanical strength is so low that the catalyst is abraded by ash particles contained in exhaust gases.